The earth moves around the sun in twelve months and absorbs sufficient energy to undergo necessary transformation.

Similarly, spiritual consciousness equips one with required energy for spiritual change. 

The northern declination of the sun starts with vernal equinox and the southern declination with the autumnal equinox.

Each declination takes six months. 

In the human physiology there are six plexus from coccygeal up to the sahasrara, to the divine cave. 

The awakened kundalini ascends and descends through the sushumna nerve, which is similar to the sun's northern and southern declination. 

In the human body the soul is the sun around which the serpentine power moves. 

In Kriya Yoga, this movement of kundalini once around sushumna nadi is equal to one years spiritual practice elsewhere.

*Excerpts from: "Kriya Yoga, The Scientific Process of Soul-Culture and the Essence of all Religions", by Swami Hariharananda Giri, Third Edition on 22 March, 1986.